Continuous process to separate colour bodies and/or asphalthenic contaminants from a hydrocarbon mixture

ABSTRACT

A continuous process to separate colour bodies and/or asphalthenic contaminants from a hydrocarbon mixture by passing part of the hydrocarbon mixture through a membrane over which membrane a pressure difference is maintained thereby obtaining a hydrocarbon permeate having a reduced content of colour bodies and/or contaminants, wherein at regular time intervals the pressure difference over the membrane is substantially lowered.

[0001] The invention is directed to a continuous process to separatecolour bodies and/or asphalthenic contaminants from a hydrocarbonmixture by passing part of the hydrocarbon mixture through a membranedue to a pressure difference across the membrane thereby obtaining ahydrocarbon permeate having a reduced content of colour bodies and/orcontaminants.

[0002] Such a process is known from WO-A-9927036. This publicationdiscloses a process for preparing lower olefins by means of thewell-known steam cracking process from a contaminated feedstock. Priorto feeding the feedstock to the steam cracker furnaces the contaminantsare removed from the feedstock by means of a membrane separation. Byremoving contaminants from the feed in this manner it is possible touse, for example, so-called black condensates as feedstock for preparinglower olefins. Black condensates are contaminated natural gascondensates having an ASTM colour of 3 or more. Direct application ofthese relatively cheap feedstocks in the above steam cracker processwould not be possible because the contaminants and/or colour bodies inthe feed would give rise to excessive coke formation in the steamcracker furnaces and associated convection sections.

[0003] A disadvantage of the process according to WO-A-9927036 is thatthe flux, expressed in feed permeating through the membrane per squaremeter per day decreased quickly from a maximum value of around forexample 1200 kg/m².day to non-economical lower values.

[0004] The object of the present invention is to provide a process,which can be operated over a prolonged time period at a high averageflux.

[0005] This object is achieved with the following process. Continuousprocess to separate colour bodies and/or asphalthenic contaminants froma hydrocarbon mixture by passing part of the hydrocarbon mixture througha membrane, over which membrane a pressure difference is maintained,thereby obtaining a hydrocarbon permeate having a reduced content ofcolour bodies and/or contaminants, wherein at regular time intervals thepressure difference over the membrane is substantially lowered.

[0006] Applicants observed that the flux would decrease from a maximumvalue to a lower value. By reducing the pressure difference when theflux reached a certain minimal acceptable value it was found possible tooperate the membrane separation at the original maximum flux when themembrane separation was resumed again. Thus a simple process wasobtained which did not require the more complex back flushing operation.Back flushing is sometimes used to improve the flux over a membrane. Adisadvantage is that it is more complex to control and it requires forexample more equipment such as back flushing pumps and will produce moreof an unwanted black by-product. Also, in the case that the membrane isformed by a thin top layer made of a dense membrane and a support layermade of a porous membrane, back flushing of permeate can cause damage ofthe thin dense membrane. Further advantages and preferred embodimentswill be described below.

[0007] The hydrocarbon mixtures will contain contaminants and/or colourbodies, which will give the hydrocarbon mixture a darkish colour. Theprocess of this invention is especially directed to hydrocarbon mixtureshaving an ASTM colour of 3 or more as determined in accordance with ASTMD1500. The ASTM colour of the permeate is found to be lower than 2 andsometimes even lower than 1, depending on the colour of the hydrocarbonfeed and operating conditions of the membrane process.

[0008] The contaminants and/or colour bodies are typically hydrocarbonswith high boiling points and which do not easily vaporise, even in thepresence of steam. Examples of such hydrocarbons are polynucleararomatics, polynuclear cycloparaffins, large paraffinic hydrocarbons(waxes), and olefinic components such as polynuclear cycloolefins andlarge olefinic hydrocarbons specially diolefins.

[0009] The hydrocarbon mixtures to be used in the process according tothe present invention are suitably hydrocarbon mixtures having aninitial boiling point of greater than 20° C. and a 95% recovery point ofless than 600° C. preferable with a 95% recovery point of less than 450°C., and more preferable a 95% recovery point of less than 350° C.determined by ASTM D-2887. Such hydrocarbon mixtures can be crudepetroleum fractions, (contaminated) natural gas condensates or(contaminated) refinery streams. An example of a suitable hydrocarbonmixture is a naphtha fraction, which has been contaminated in thestorage tank or in the pipeline when transporting said fraction from arefinery to a steam cracker. Another example of a hydrocarbon mixture,which may suitably be used, is the above referred to black condensate,which is a contaminated natural gas condensate. The natural gascondensates normally have a ASTM colour of below 1. Contamination occurswhen such gas condensates are stored in storage vessels or transportedvia pipelines through which also, for example, crude oils arestored/transported. Natural gas condensates are typically mixturescomprising substantially, i.e. more than 90 wt %, of C₅ to C₂₀hydrocarbons or more typically C₅ to C₁₂ hydrocarbons.

[0010] The membrane suitably comprises a top layer made of a densemembrane and a base layer (support) made of a porous membrane. Themembrane is suitably so arranged that the permeate flows first throughthe dense membrane top layer and then through the base layer, so thatthe pressure difference over the membrane pushes the top layer onto thebase layer. The dense membrane layer is the actual membrane whichseparates the contaminants from the hydrocarbon mixture. The densemembrane, which is well known to one skilled in the art, has propertiessuch that the hydrocarbon mixture passes said membrane by dissolving inand diffusing through its structure. Preferably the dense membrane layerhas a so-called cross-linked structure as for example described inWO-A-9627430. The thickness of the dense membrane layer is preferably asthin as possible. Suitably the thickness is between 1 and 15 micrometer,preferably between 1 and 5 micrometer. The contaminants and colourbodies are not capable to dissolve in said dense membrane because oftheir more complex structure and high molecular weight. For example,suitable dense membranes can be made from a polysiloxane, in particularfrom poly(di-methyl siloxane) (PDMS). The porous membrane layer providesmechanical strength to the membrane. Suitable porous membranes arePolyAcryloNitrile (PAN), PolyAmideImide+TiO₂ (PAI) andPolyEtherImide(PEI), and can be of the type commonly used forultrafiltration, nanofiltration or reverse osmosis.

[0011] The process according to the invention comprises first timeperiods at which the actual separation takes place and a high flux isachieved, alternated with second time periods at which the pressuredifference over the membrane is substantially lowered when compared tothe first time periods. After the second time periods it was foundpossible to operate the membrane separation at substantially theoriginal high flux again, without significant deterioration overprolonged times of operation. Without wanting to limit the invention inany manner, it is believed that the following mechanism contributes toprevent degrading membrane performance due to deposits of colour bodiesand/or asphaltenic contaminants on the membrane surface. Duringoperation, the dense membrane is swollen significantly, due to thehydrocarbon that is dissolved in and diffusing through the membrane.I.e. the thickness of the dense membrane is increased during operation,although the swelling is somewhat counteracted by the pressuredifference over the membrane. When the pressure difference issignificantly lowered, it is believed that the dense membrane can expandso that its thickness increases, thereby loosening any deposits on themembrane surface.

[0012] During separation the pressure difference across the membrane ispreferably between 5 and 60 bar and more preferably between 10 and 30bar. During the time interval at which the pressure difference islowered the pressure difference is preferably between 0 and 5 bar, morepreferably below 1 bar and most preferably 0 bar.

[0013] The pressure difference can be suitably achieved by operatingpumping means upstream and/or downstream the membranes. In a preferredembodiment of the invention the lowering of the pressure at regularintervals is achieved by stopping the flow of contaminated hydrocarbonmixture to the membrane. This can be achieved by stopping the pumpingmeans. Stopping and activating pumping means is not always desirable. Ina situation wherein the pressure difference is achieved by at least anupstream pump it can be desirable to recycle the hydrocarbon mixturefrom a position between the operating pump and the membrane to aposition upstream the operating pump without stopping the pump. In thismanner the flow to the membrane can be temporarily discontinued whilethe pump can remain in its operating mode. Alternatively one upstreampumping means can provide a hydrocarbon mixture feed to more than oneparallel operating membrane separator or one or more parallel operatinggroups of parallel operating membrane separators, each separator orgroup of separators provided with an individual valve to interrupt thefeed to said separator or group of separators. By closing and opening ina sequential manner the separate valves the (groups of) membraneseparators can be operated according to the process of the presentinvention without having to stop the upstream pump.

[0014] The above-described and parallel-operated (groups of) separatorscomprise a single separation step. Embodiments comprising two or moresequential separation steps, wherein the retentate of a first separationstep is used as the feed for a second separation step, are preferred.

[0015] One skilled in the art can easily determine the optimal timeperiods of continuous separation and the time periods at which thepressure difference is substantially lower. Maximising the average fluxover the membrane separator will drive such determination. With averageflux is here meant the average flux over both separation andintermediate time periods. Thus it is desirable to minimise the timeperiods at which the pressure is substantially lower and maximising thetime period at which separation takes place. The flux will decrease inthe separation intervals and suitably when the flux becomes less than75-99% of its maximum value the separation interval is stopped. Suitablybetween 5 and 480 minutes of continuous separation across the membranealternates with time periods of between 1 and 60 minutes, preferablybelow 30 minutes and more preferably below 10 minutes and mostpreferably below 6 minutes of at which the pressure difference issubstantially lowered.

[0016] The membrane separation is suitably carried out at a temperaturein the range of from −20 to 100° C., in particular 10 to 100° C., andsuitably at 40° C. The wt % recovery of permeate on feed is preferablybetween 50 and 97 wt % and more preferably between 80 and 95 wt %.

[0017] The process according to the invention is suitable to be used toseparate contaminants from a feed, especially the referred to blackcondensates, for a steam cracker of which WO-A-9927036 describes anexample. The retentate which contains an increased concentration ofcontaminants may be supplied to the fractionation column downstream thesteam cracker furnaces. Preferably the retentate is supplied to a crudedistillation column of a refinery because the various components of theretentate are also found in the crude petroleum feedstock normallysupplied to said crude distillation column.

[0018] The invention will be described by means of the followingnon-limiting examples.

EXAMPLE 1

[0019] A black condensate having the properties as listed in Table 1 wasfed at a rate of 100 kg/hour to a membrane separation unit at 40° C.which was provided with 1.5 m² of a PDMS/PAN 150 membrane as obtainedfrom GKSS Forschungszentrum GmbH (a company having its principal officein Geesthacht, Germany) comprising a top layer of PolyDiMethylSiloxane(PDMS) and a supporting layer of a PolyAcryloNitrile (PAN). The pressuredifference when separating was 25 bar. This pressure was alternated byperiods at which the feed was stopped resulting in a pressure differenceof 0 bar. Experiment 1 was started at an operation time readout of 740hours, and the flux across the membrane during the first 2.5 hours ofthe experiment was as shown in FIG. 1. The colour properties of thepermeate was an ASTM colour of less than 2.

[0020] The procedure as described above was subsequently continued formore than 400 hours, and the flux across the membrane vs. time remainedthe same as in the first 2.5 hours shown in FIG. 1. The average fluxtherefore did not decrease. TABLE 1 properties black condensate densityat 15° C., kg/m3 750 IBP ° C. <36 95 vol % BP ° C. 302 ASTM Colour (ASTMD1500) 8

COMPARATIVE EXAMPLE

[0021] Example 1 can be compared with the results of a similar testcampaign wherein the same membranes and the same condensate feed wasused. The feed differential pressure over the membrane was 20 bar,temperature 40° C., feed rate 80 kg/h. The flux across the membrane inthe comparative example is shown in FIG. 2. The comparative example wasstarted at an operation time readout of 120 hours. The flux reduced tosome 50% of the initial flux during the first 200 hours of operation.The flux did not reach a steady state condition.

[0022] This Experiment shows that even with a lower pressuredifferential over the membrane a decrease in the flux is experiencedwhen the on-off mode of the process of the present invention is notapplied. If this Experiment was performed under the same pressure andfeed rate conditions as Example 1 an even quicker decrease in flux wouldhave been observed.

[0023] As briefly pointed out in the introduction, the present inventioncan for example be used with particular advantage in a process ofproducing light olefins from a liquid hydrocarbon feed by means ofthermal cracking, as known from WO-A-9927036. The known processcomprises the steps of

[0024] (a) supplying the feed to the inlet of a membrane unit providedwith a membrane, and removing from the permeate side a permeate and fromthe retentate side a retentate;

[0025] (b) supplying the permeate to the inlet of a cracking furnace,allowing the permeate to crack in the coils of the cracking furnace inthe presence of steam at elevated temperature and removing from thecracking furnace a cracked stream which is enriched in light olefins;

[0026] (c) quenching the cracked stream;

[0027] (d) supplying the cooled cracked stream to a fractionationcolumn;

[0028] (e) supplying the retentate to the fractionation column; and

[0029] (f) removing from the top of the fractionation column a gaseousstream, from the side of the fractionation column a side stream of fueloil components and from the bottom of the fractionation column a bottomstream.

[0030] Such a process is also called steam cracking, naphtha cracking orethylene manufacturing.

[0031] The fractionation column is also called ‘primary fractionator’.

[0032] The gaseous stream removed from the top of the fractionationcolumn comprises light olefins, such as ethylene and propylene, andother components, such as hydrogen, methane, C4 products and pyrolysisgasoline (C5+). Downstream of the fractionation column, the gaseousoverhead is further treated to recover ethylene.

[0033] From the side of the fractionation column one or more sidestream(s) is (are) removed which contains fuel oil components.

[0034] From the bottom of the fractionation column is removed a liquidbottom stream which contains heavy cracked fuel oil. Part of the liquidbottom stream is cooled and mixed with the cracked stream upstream ofthe fractionation column to quench this stream. The remainder is removedas heavy fuel oil.

[0035] Upstream of the fractionation column the feed is cracked in thecracking furnace. The liquid hydrocarbon feed is preheated upstream ofthe cracking furnace or inside the upper part of the cracking furnace.In the cracking furnace the liquid hydrocarbon stream is first vaporizedand subsequently cracked. Vaporization of the liquid hydrocarbon streamtakes place in the presence of steam in a vaporization coil located inthe upper part of the cracking furnace, where the liquid is vaporized bythe heat from the hot flue gas. The upper part of the cracking furnaceis called the convection section. After the stream is vaporized, itenters into the pyrolysis coil in the radiant section of the crackingfurnace. In the pyrolysis coil hydrocarbons are cracked in the presenceof steam to obtain the desired product. This is well known, and theconditions for vaporization and cracking are well known as well.

[0036] Feeds that are used are naphtha (a straight-run gasolinefraction) and/or gas oil (a distillate, intermediate in characterbetween kerosene and light lubricating oils). Such feeds, however, tendto become more expensive, and this triggers the interest in using otherhydrocarbon feeds for the cracking process. Examples of such feeds arecertain condensates which comprise naphtha and gas oil components.Condensate is a mixture of hydrocarbons which are sometimes producedwith natural gas.

[0037] These feeds, however, also contain contaminants. Two contaminantsare of particular relevance. On the one hand hydrocarbons with a highboiling point and on the other hand salts present in water dropletswhich are dispersed in the stream of light hydrocarbons.

[0038] Hydrocarbons with a high boiling points are hydrocarbons which donot easily vaporize, even in the presence of steam. Examples of suchhydrocarbons are polynuclear aromatics, polynuclear cycloparaffins,large paraffinic hydrocarbons (waxes), and olefinic components such aspolynuclear cycloolefins and large olefinic hydrocarbons speciallydiolefins. These high boiling point hydrocarbons are soluble in thelight hydrocarbons, and the solution usually has a darker colour forexample an ASTM colour of 3 or more, determined in accordance with ASTMD1500. An example of a contaminated liquid stream containing lighthydrocarbons is a black condensate, which is a mixture of hydrocarbonswhich are sometimes produced with natural gas having an ASTM colour of 3or more. The contaminated liquid may also include waste streams for therefinery.

[0039] The salts in the hydrocarbon streams will come from formationwater or from other treatments at a refinery, examples of contaminatingsalts are sodium chloride, magnesium chloride, calcium chloride and ironchloride. Other salts, such as sulphates may be present as well.

[0040] The membrane separation step of the known process serves toremove the contaminants from the feed. If the contaminants are notremoved, they will remain liquid in the vaporization coil, and will foulthe inner surface of the vaporization coil. Fouling by depositedcomponents will reduce the heat transfer and will consequently adverselyaffect the performance of a steam cracker. Moreover, fouling can evencause plugging of the vaporization coil. In the known process thereforefouling of the vaporization coil is reduced.

[0041] The known process can be improved by making use of the presentinvention in order that it can be operated over a significantlyprolonged time period at a high average flux.

[0042] Using the present invention this can be achieved by replacing thefeed supply and membrane separation step of the known process by thestep of supplying the feed to the inlet of a membrane unit provided witha membrane, over which membrane a pressure difference is maintainedthereby obtaining at the permeate side of the membrane a permeate havinga reduced content of colour bodies and/or contaminants, and at theretentate side of the membrane a retentate, and removing the permeateand the retentate from the membrane, wherein at regular time intervalsthe pressure difference over the membrane is substantially lowered.

[0043] Accordingly, the present invention further provides a processaccording to any one of claims 1-13, wherein the hydrocarbon mixture isa liquid hydrocarbon feed from which light olefins are to be produced bythermal cracking, wherein the membrane forms part of a membraneseparation unit in which the hydrocarbon permeate is removed from thepermeate side of the membrane, and wherein a retentate is removed fromthe retentate side of the membrane, and wherein the process furthercomprises the steps of

[0044] (a) supplying the permeate to the inlet of a cracking furnace,allowing the permeate to crack in the coils of the cracking furnace inthe presence of steam at elevated temperature and removing from thecracking furnace a cracked stream which is enriched in light olefins;

[0045] (b) quenching the cracked stream;

[0046] (c) supplying the cooled cracked stream to a fractionationcolumn;

[0047] (d) supplying the retentate to the fractionation column; and

[0048] (e) removing from the top of the fractionation column a gaseousstream, from the side of the fractionation column a side stream of fueloil components and from the bottom of the fractionation column a bottomstream.

[0049] Suitably, the membrane in step (a) comprises a dense membranelayer as described hereinbefore, which allows hydrocarbons from thefeed, but not asphaltenes or color bodies to pass through the membraneby dissolving in and diffusing through its structure. Such a membrane issuitably also used, when the hydrocarbon feed further contains saltcontaminants, which are present in water droplets that are dispersed inthe hydrocarbon feed. The water and/or salt will normally not bedissolved in the dense membrane, and therefore the permeate will be freefrom salt.

[0050] The membrane separation is carried out at a temperature in therange of from 10 to 100° C. and suitably at 40° C., and the mass ratiobetween permeate and retentate is between 1 and 100′, and suitablybetween 5 and 20. Examples of further details about the operation of themembrane are given in the description hereinbefore and in Example 1.Details about the cracking process are given in the example disclosed inWO-A-9927036.

1. A continuous process to separate colour bodies and/or asphaltheniccontaminants from a hydrocarbon mixture by passing part of thehydrocarbon mixture through a membrane over which membrane a pressuredifference is maintained thereby obtaining a hydrocarbon permeate havinga reduced content of colour bodies and/or contaminants, wherein atregular time intervals the pressure difference over the membrane issubstantially lowered.
 2. The process of claim 1, wherein the membranecomprises a top layer made of a dense membrane and a support layer madeof a porous membrane.
 3. The process of claim 2, wherein the densemembrane is made from a polysiloxane such as a poly(di-methyl siloxane).4. The process of claims 3, wherein the pressure difference across themembrane is between 5 and 60 bar during separation and between 0 and 5bar when the pressure difference is lowered at the regular timeintervals.
 5. The process of claim 4, wherein the pressure differenceacross the membrane during separation is between 10 and 30 bar.
 6. Theprocess of claims 5, wherein at regular time intervals the pressuredifference is lowered to 0 bar.
 7. The process of claim 6, wherein timeperiods of between 5 and 480 minutes of continuous separation across themembrane alternates with time periods of between 1 and 60 minutes of atwhich the pressure difference is substantially lowered.
 8. The processclaims 7, wherein the time period at which the pressure difference issubstantially lowered is below 30 minutes.
 9. The process of claim 8,wherein the pressure difference is substantially lowered by stopping theflow to the membrane.
 10. The process of claim 9, wherein at least partof the pressure difference across the membrane results from a pumpupstream the membrane and wherein the pressure difference is lowered atregular time intervals by recycling the hydrocarbon mixture from aposition between the operating pump and the membrane to a positionupstream the operating pump.
 11. The process of claim 10, wherein thehydrocarbon mixture has an initial boiling point greater than 20° C. anda 95% recovery point of less than 600° C., determined by ASTM D2887. 12.The process of claim 11, wherein the hydrocarbon mixture has an ASTMcolour of above 3 according to ASTM D1500.
 13. The process claim 12,wherein the hydrocarbon mixture is a contaminated natural gas condensateor a contaminated refinery stream.
 14. The process of claim 13, whereinthe hydrocarbon mixture is a liquid hydrocarbon feed from which lightolefins are to be produced by thermal cracking, wherein the membraneforms part of a membrane separation unit in which the hydrocarbonpermeate is removed from the permeate side of the membrane, and whereina retentate is removed from the retentate side of the membrane, andwherein the process further comprises the steps of (a) supplying thepermeate to the inlet of a cracking furnace, allowing the permeate tocrack in the coils of the cracking furnace in the presence of steam atelevated temperature and removing from the cracking furnace a crackedstream which is enriched in light olefins; (b) quenching the crackedstream; (c) supplying the cooled cracked stream to a fractionationcolumn; (d) supplying the retentate to the fractionation column; and (e)removing from the top of the fractionation column a gaseous stream, fromthe side of the fractionation column a side stream of fuel oilcomponents and from the bottom of the fractionation column a bottomstream.
 15. The process of claim 1, wherein the pressure differenceacross the membrane is between 5 and 60 bar during separation andbetween 0 and 5 bar when the pressure difference is lowered at theregular time intervals.
 16. The process of claim 2, wherein the pressuredifference across the membrane is between 5 and 60 bar during separationand between 0 and 5 bar when the pressure difference is lowered at theregular time intervals.
 17. The process of claim 1, wherein at regulartime intervals the pressure difference is lowered to 0 bar.
 18. Theprocess of claim 2, wherein at regular time intervals the pressuredifference is lowered to 0 bar.
 19. The process of claim 3, wherein atregular time intervals the pressure difference is lowered to 0 bar. 20.The process of claim 4, wherein at regular time intervals the pressuredifference is lowered to 0 bar.
 21. The process-of claim 1, wherein timeperiods of between 5 and 480 minutes of continuous separation across themembrane alternates with time periods of between 1 and 60 minutes of atwhich the pressure difference is substantially lowered.
 22. Theprocess-of claim 2, wherein time periods of between 5 and 480 minutes ofcontinuous separation across the membrane alternates with time periodsof between 1 and 60 minutes of at which the pressure difference issubstantially lowered.
 23. The process-of claim 3, wherein time periodsof between 5 and 480 minutes of continuous separation across themembrane alternates with time periods of between 1 and 60 minutes of atwhich the pressure difference is substantially lowered.
 24. Theprocess-of claim 4, wherein time periods of between 5 and 480 minutes ofcontinuous separation across the membrane alternates with time periodsof between 1 and 60 minutes of at which the pressure difference issubstantially lowered.
 25. The process-of claim 5, wherein time periodsof between 5 and 480 minutes of continuous separation across themembrane alternates with time periods of between 1 and 60 minutes of atwhich the pressure difference is substantially lowered.
 26. The processof claim 1, wherein the pressure difference is substantially lowered bystopping the flow to the membrane.
 27. The process of claim 2, whereinthe pressure difference is substantially lowered by stopping the flow tothe membrane.
 28. The process of claim 3, wherein the pressuredifference is substantially lowered by stopping the flow to themembrane.
 29. The process of claim 4, wherein the pressure difference issubstantially lowered by stopping the flow to the membrane.
 30. Theprocess of claim 5, wherein the pressure difference is substantiallylowered by stopping the flow to the membrane.
 31. The process of claim6, wherein the pressure difference is substantially lowered by stoppingthe flow to the membrane.
 32. The process of claim 7, wherein thepressure difference is substantially lowered by stopping the flow to themembrane.